1. Introduction - The Mid-Tertiary Ignimbrite Flare-Up

Coney (1978) described the mid-Tertiary ignimbrite flare-up of the southern Cordilleran as a "most fascinating phenomenon". Johnson (1991) estimated that over 500,000 km3 of ash flow tuffs were erupted in middle to late Cenozoic time, and 5,000,000 km3 of intermediate- to silicic- magmas may have been associated with explosive volcanism. The flare-up event was comprised of eruptions of lavas and ignimbrites. Whereas a lava cools from magma, an ignimbrite is a pyroclastic flow, consisting of glassy fragments welded together, that flows away from the volcano (Thorpe and Brown, 1985). Pyroclastic flows are comprised of rock fragments created during explosive volcanism. Grain sizes can range from ash (less than 2 mm diameter) to lapilli (2 to 64 mm diameter) to block and bomb size (greater than 64 mm diameter). Ignimbrites may display depositional structures, such as laminations and bedding. Other common terms for ignimbrites include rocks displaying welded fabrics, such as welded tuffs and welded ash-flow tuffs. Since not all pyroclastic flows are welded, the phrase "all ignimbrites are pyroclastic flows, but not all pyroclastic flows are ignimbrites" holds true.

This investigation focuses on three aspects of the flare-up event. First, identifying the source of the parent magma for the lavas and ignimbrites is crucial. Tectonic settings may produce different magma compositions, for example, a mid-ocean ridge basalt as opposed to a calc-alkaline volcanic from a subduction zone. Second, magmatic processes during transport may influence the composition of lavas and tuffs. Partial melting and fractional crystallization are the most significant processes but several other processes (i.e., magma mixing and contamination) may be important. Third, the relationship between magmatism and tectonics is a complex story for the flare-up event. Although by no means solved, controversies and current theories regarding the tectonic environment of the flare-up are presented. Use the links below to advance to the next section, visit the previous page, bring up references, or return to the main page.

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